1. Field of the Invention
This invention relates to the treatment of a gaseous exhaust stream, and more specifically to a method and apparatus for treating an engine exhaust stream containing pollutants including hydrocarbons both during a cold-start period and a subsequent operating period.
2. Related Art
Molecular sieves, including zeolites, are known for adsorbing hydrocarbons from gas streams such as engine exhaust in exhaust gas purification schemes which also include contacting the exhaust gas with a catalyst. For example, U.S. Pat. No. 4,985,210 to Minami, dated Jan. 15, 1991, is directed to an exhaust gas purifying apparatus for an automobile employing a three-way catalyst with either a Y-type zeolite or a mordenite used in a hydrocarbon trap upstream of the three-way catalyst. A bed of activated carbon may be disposed upstream of the adsorbent zone and a valve mechanism serves to by-pass the activated carbon bed at certain temperatures of the exhaust gas stream.
U.S. Pat. No. 5,051,244 to Dunne et al, dated Sep. 24, 1991, concerns passing an engine exhaust stream through a molecular sieve to adsorb hydrocarbons during the cold-start phase of engine operation. As the engine heats up and the hydrocarbons begin to desorb, the molecular sieve is by-passed until a catalyst is at its operating temperature, at which point the gas stream is again flowed through the molecular sieve to desorb hydrocarbons and carry them to the catalyst zone. A paper by M. Heimrich, L. Smith and J. Kotowski entitled Cold-Start Hydrocarbon Collection for Advanced Exhaust Emission Control, SAE Publication Number 920847, discloses an apparatus which functions in a manner similar to that of U.S. Pat. No. 5,051,244.
U.S. Pat. No. 5,125,231 to Patil et al, dated Jun. 30, 1992, discloses an engine exhaust system for reducing hydrocarbon emissions, including the use of Beta-zeolites as hydrocarbon adsorbents (see column 5, lines 63 through column 6, line 14). Zeolites having a silica/alumina ratio in the range of 70/1 to 200/1 are preferred adsorbents (see column 6, lines 7-11). The apparatus provides for by-passing a first converter during cold-start operation and, when the first converter reaches its light-off temperature, either by-passing the second converter or recycling effluent from it to the first converter (see e.g., column 10, lines 10-28).
U.S. Pat. No. 2,942,932 to Elliott, dated Jun. 28, 1960, discloses the use of an adsorbent bed to adsorb hydrocarbons in an exhaust gas stream, which hydrocarbons are desorbed as the exhaust gas heats up. The effluent from the adsorbent bed is mixed with air in an eductor and fed to an oxidation catalyst zone.
U.S. Pat. No. 3,699,683 to Tourtellotte et al, dated Oct. 24, 1972, discloses a system for the prevention of hydrocarbon emissions from engines during start-up, in which a reducing catalyst bed, an oxidizing catalyst bed and an absorbent bed are positioned in series for treatment of the engine exhaust. An air blower supplies air between the reducing catalyst bed and the oxidizing catalyst bed and a valve system is provided to permit by-passing the absorbent bed which is regenerated by passing a small stream of hot exhaust gas through it.
U.S. Pat. No. 5,158,753 to Take et al, dated Oct. 27, 1992, discloses an engine exhaust gas purifying apparatus in which the engine exhaust is passed through the hot side of a heat exchanger, then through a hydrocarbon adsorption zone, the effluent of which is passed through the cold side of the heat exchanger in heat exchange with the hot exhaust gases emanating from the engine and then passed into a catalyst. In one embodiment, a catalyst is contained on the cold side of the heat exchanger.